Inosine monophosphate dehydrogenase (IMPDH) catalyzes the oxidation of inosine monophosphate (IMP) to xanthosine monophosphate (XMP) with the concomitant reduction of NAD+. This reaction controls the entry of purines into the guanine nucleotide pool, making IMPDH a billion-dollar target for immunosuppressive, antiviral and anticancer drugs. The IMPDH reaction involves two different chemical transformations: hydride transfer and hydrolysis of a covalent intermediate, and thus provides unique opportunity to investigate how proteins efficiently change conformations to accommodate multiple substrate binding events, transition states and product release steps. This enzyme displays several remarkable mechanistic features, including a large conformational change in mid-catalytic stream, an Arg residue that acts as a general base catalyst and a K+ that acts as a molecular lubricant. Lastly, IMPDH contains two CBS domains of unknown function (named for the related domains in cystathionine (3 synthase). Mutations in the CBS domains of IMPDH1 cause autosomal dominant retinitis pigmentosa, an inherited blindness, yet have no effect on the enzymatic activity of IMPDH. Thus IMPDH must have another cellular function. We propose to address the following questions: Aim 1. How is water activated? Aim 2. How does IMPDH remodel to accommodate different transition states? Aim 3. What is the mechanism of K+ activation? Aim 4. What is the function of the CBS domains? This work will provide important new insights into IMPDH function that will be invaluable in the design of new cancer, viral and immunosuppressive therapy. Elucidation of the function of the CBS domains is critical for the development of chemotherapy of retinitis pigmentosa and other inherited diseases involving CBS domains.